The present invention relates to a method of producing a ceramic matrix composite requiring little energy consumption for production, and a ceramic matrix composite obtained by the method for production.
A composite material is a composition aggregate in which a plurality of raw-materials are macroscopically mixed to provide characteristics, which a raw material alone could not realize, by complementarily utilizing mechanical properties each raw material possesses. Basically, the method of producing a composite material is a technical method by which a material is combined with other material, and there are various combinations depending on matrixes and dispersed material (reinforcements etc.), intended purposes, or cost and the like.
Since ceramic matrix composite (referred to also as xe2x80x9cCMCxe2x80x9d hereinafter) and intermetallic matrix composite (referred to also as xe2x80x9cIMCxe2x80x9d hereinafter) among them have physical characteristics that a metal matrix composite (referred to also as xe2x80x9cMMCxe2x80x9d hereinafter) does not have, such as excellent heat resistance, utilization in various industrial fields has been intended.
Especially, aluminum nitride (AlN), a kind of ceramic, attracts attention taking advantage of characteristics, such as outstanding heat conduction characteristics and low coefficient of thermal expansion, as material for a high thermal conductivity substrate or a member for semiconductor manufacturing equipment. However, aluminum nitride may belong to a category demonstrating a low fracture toughness value among ceramic materials, and crack may occur under a service condition with loads, such as thermal shock, or when being combined with dissimilar materials, to impede the above described characteristics. For this reason, production of composite materials is investigated to improve fracture toughness value, etc. As a process of synthesizing a TiB2/AlN composite material that is a similar material to that in the present invention, G. J. Zhang et al. studied an increase in strength and improvement of physical properties of aluminum nitride, using Al, TiH2, and BN powder and utilizing a reactive sintering method to induce a reaction between elements (Ceramics International, 22(1996), 143). However, in an in-situ CMC production process like this technique, synthesis under conditions of high pressure and high temperature is required as in conventional methods, and since near net shaping is also difficult, this process is accompanied with high cost. For this reason, a sintering process in which pressurized sintering is performed under a high temperature condition is needed as a general method for production of aluminum nitride and CMC utilizing this as a matrix.
And, for example, a method for production of intermetallic compound and ceramic is disclosed, in which raw material powder is mixed to produce a green compact, and then reaction is carried out by firing the green compact, in Japanese Patent No. 2609376 as specific related technology.
On the other hand, National Publication of International Patent Application No. 1996-508460 discloses, a method of producing a composite material by synthesis with firing under a low pressure of gaseous nitrogen using Al and boride or carbide of a transition metal as raw materials.
Generally, in a method for production of CMC which uses ceramic, such as aluminum nitride as matrix, in which pressurized sintering is carried out under a condition at a high temperature, special pressurizing devices and production instruments are required and there is a problem that manufacturing cost becomes high. Besides aluminum nitride as a raw material powder used for sintering has a necessity of being synthesized by techniques, such as reduction nitriding method and direct nitriding method, aluminum nitride sintered compact also shows difficulty in sintering, thus this technique becomes very complicated manufacturing process, and furthermore, sintering process at elevated temperature up to about no less than 1700xc2x0 C. itself requires excessive energy consumption.
Among methods for production of the above described aluminum nitride raw material powder are a reduction nitriding method in which nitriding is performed by nitrogen gas or ammonia gas while Al2O3 powder having high grade is reduced with carbon, and a direct nitriding method in which aluminum powder is nitrided by nitrogen gas or ammonia gas. However, in the reduction nitriding method, since reaction itself is an endothermic reaction while there is an advantage of being able to obtain high grade aluminum nitride powder, a process requiring a large amount of energy is needed. And while the direct nitriding method is economical process using exothermic reaction, since grain size obtained by synthesis is coarse, further pulverizing process is needed. When sintering process is regarded as a material production process, even if only process for synthesizing raw material powder is taken into consideration, it may probably be regarded as a process with extraordinary high energy consumption. Besides, since external energy by heating with heater is utilized to heat a furnace atmosphere, maintaining of an elevated temperature in case of sintering ceramic shows an inferior thermal efficiency, and therefore it is a process with a very large energy loss.
And, according to a method for production shown in Japanese Patent No. 2609376 official report, in order to a manufacture composite material that has a densified fine structure there is a necessity to completely melt a formed matrix. Therefore, restrictions arise in performance and scale of manufacturing apparatus, and there is a problem that production of a composite material that is large-sized or has a complicated form is very difficult, and it is difficult to perform near net shaping in view of a form of a final product simultaneously. From this point of view, a case may be assumed in which increase in processing cost in next process may be caused when it is taken into consideration that the ceramic and the composite material itself are material having a difficult workability.
Furthermore, in the method for production shown in National Publication of International Patent Application No. 1996-508460 official report, in order to reduce an amount of Al remained unavoidably, there is a necessity that the reaction is fully progressed. However, since strict control of material composition ratio, reaction conditions, etc. were required for completion of the reaction, it was difficult technically and in manufacturing cost to reduce Al residual percentage.
The present invention has been done in view of these problems associated with conventional arts and aims at a method of producing a ceramic matrix composite produced by a method, which production method reduces metal residual percentage within matrix by little energy consumption, without requiring special external heating means and special equipment while it is industrially simple and at a low price, and a ceramic matrix composite obtained by this method for production having a high strength and low thermal expansion characteristic.
That is, according to the present invention, a method of producing a ceramic matrix composite shown below and a ceramic matrix composite produced by the method for production are provided.
(1) A method of producing a ceramic matrix composite comprising the steps of: filling mixed powder obtained by mixing metal powder and boron nitride powder into a predetermined container to form a green compact having a porous structure; and infiltrating the green compact with molten Al to form a composite material containing metal boride and having aluminum nitride as a matrix, wherein the green compact is formed by compressing the mixed powder whose mixing ratio of the metal powder to the boron nitride powder is 1:1.8 to 1:2.2 (molar ratio) so that porosity of the green compact is 34 to 42%.
(2) A method of producing a ceramic matrix composite comprising the steps of: filling mixed powder obtained by mixing metal powder and boron nitride powder into a predetermined container to form a green compact having a porous structure; and infiltrating the green compact with molten Al to form a composite material containing metal boride and having aluminum nitride as a matrix, wherein the green compact is formed by compressing the mixed powder whose mixing ratio of the metal powder to the boron nitride powder is 1:2.2 to 1:4.0 (molar ratio) so that porosity of the green compact is 26 to 40%.
(3) A method of producing a ceramic matrix composite comprising the steps of: filling mixed powder obtained by mixing metal powder and boron nitride powder into a predetermined container to form a green compact having a porous structure; and infiltrating the green compact with molten Al to form a composite material containing metal boride and having aluminum nitride as a matrix, wherein the green compact is formed by compressing the mixed powder whose mixing ratio of the metal powder to the boron nitride powder is 1:1.8 to 1:2.2 (molar ratio) is compressed so that porosity of the green compact is 15 to 34%.
(4) A method of producing a ceramic matrix composite comprising the steps of: filling mixed powder obtained by mixing metal powder and boron nitride powder into a predetermined container to form a green compact having a porous structure; and infiltrating the green compact with molten Al to form a composite material containing metal boride and having aluminum nitride as a matrix, wherein the green compact is formed by compressing the mixed powder whose mixing ratio of the metal powder to the boron nitride powder is 1:2.2 to 1:4.0 (molar ratio) is compressed to form the green compact so that porosity of the green compact is 15 to 26%.
In the method of producing a ceramic matrix composite according to the present invention, the mixed powder may further contain aluminum nitride particles in a ratio of not more than 1 mole to 1 mole of the metal powder in addition to the metal powder and the boron nitride powder.
It is preferable to use boron nitride powder having a mean particle diameter of not more than 20 xcexcm.
It is also preferable to maintain the green compact at 1000 to 1400xc2x0 C. for no less than 10 minutes after infiltration with molten Al.
It is preferred to use further at least one kind of the metal powder selected from the group consisting of Ti, Ta, Hf, Nb, and Zr.
In the present invention, the infiltration with molten Al is preferably carried out under inert gas atmosphere.
A molten Al may preferably contain not more than 3% by mass of Mg.
There is further provided a ceramic matrix composite produced by the method of producing a ceramic matrix composite according to the present invention, which comprises metal boride, and a matrix having as a principal component aluminum nitride whose Al content is not more than 20% by volume.
It is preferred that the Al content in aluminum nitride is not more than 10% by volume.
There is still further provided a ceramic matrix composite produced by the method of producing a ceramic matrix composite according to the present invention, which comprises metal boride, boron nitride, and a matrix having as a principal component aluminum nitride whose Al content is not more than 20% by volume.
In this case, it is preferable that the Al content in aluminum nitride is not more than 10% by volume. Further, it is preferable that the ceramic matrix composite has a thermal expansion coefficient of 10 ppm/K or less.